Coping with NE5532D output DC offset

VEC7OR · 17th September 2005, 08:39 PM

I've designed and assembled a headphone amplifier based on NE5532D (http://images.bite.lt/banga/files/cl...ac13923b17.png schematics, http://images.bite.lt/banga/files/cl...ac189633db.jpg PCB, http://images.bite.lt/banga/files/cl...ac1d14c818.jpg insides, http://images.bite.lt/banga/files/cl...ac1e4eedd1.jpg front)

On each output it has 0.121V DC offset, I dont think it does alot of harm to my 64ohm headphones because power sink due to dc offset is just ~220uW, but it somewhat bugs me, how can I rid of it ?

P.S. As for the amp itself I really like the sound - its dead quiet when there is no signal and transparent when it is

.

peranders · 17th September 2005, 08:59 PM

You have made one or two mistakes.

The offset (taken from my head) is 1-4 mV times your gain, 11 in your case (a bit much gain?) this gives you approx. 40-50 mV. The reason why you have so much additional offset is the 100 k at the input. If you want to minimize the offset you should insert a capacitor between 10 k and ground. This will create an offset of 4-10 mV only. The trick is that each input of the opamp should "see" the same DC resistance.

VEC7OR · 17th September 2005, 09:12 PM

Or I just remove 100k resistor ?

mzzj · 17th September 2005, 10:00 PM

Quote:

Originally posted by VEC7OR
Or I just remove 100k resistor ?

Noo!

you have to aim for equal (dc) resistances in both inputs, if you remove that 100k then you end up with far worse situation as then its 10k||100k(~9k) for -input and infinite for + input.

peranders · 18th September 2005, 07:18 AM

Quote:

Originally posted by VEC7OR
Or I just remove 100k resistor ?

No, because every opamp MUST have a DC path somewhere in order to work. The NE5532 has NPN transistors at the input and those transistors must get base current from somewhere.

VEC7OR · 21st September 2005, 01:31 PM

I think I got it now, I should replace 100k resistor in input to 9k resistor, that way both inputs will see the ground through 10k resistance.

17th September 2005, 08:39 PM	#1
VEC7OR diyAudio Member Join Date: Jul 2003 Location: Vilnius	Coping with NE5532D output DC offset I've designed and assembled a headphone amplifier based on NE5532D (http://images.bite.lt/banga/files/cl...ac13923b17.png schematics, http://images.bite.lt/banga/files/cl...ac189633db.jpg PCB, http://images.bite.lt/banga/files/cl...ac1d14c818.jpg insides, http://images.bite.lt/banga/files/cl...ac1e4eedd1.jpg front) On each output it has 0.121V DC offset, I dont think it does alot of harm to my 64ohm headphones because power sink due to dc offset is just ~220uW, but it somewhat bugs me, how can I rid of it ? P.S. As for the amp itself I really like the sound - its dead quiet when there is no signal and transparent when it is . __________________ Profanity took a very interesting form in Russia where there exists a language of sorts, most of its words based on four basic profane roots - nouns p...is, wh.0.e, c..t and verb f..k. It is possible to sensibly communicate using just these four basic roots.

17th September 2005, 08:59 PM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	You have made one or two mistakes. The offset (taken from my head) is 1-4 mV times your gain, 11 in your case (a bit much gain?) this gives you approx. 40-50 mV. The reason why you have so much additional offset is the 100 k at the input. If you want to minimize the offset you should insert a capacitor between 10 k and ground. This will create an offset of 4-10 mV only. The trick is that each input of the opamp should "see" the same DC resistance. __________________ /Per-Anders (my first name) or P-A as my friends call me

17th September 2005, 09:12 PM	#3
VEC7OR diyAudio Member Join Date: Jul 2003 Location: Vilnius	Or I just remove 100k resistor ? __________________ Profanity took a very interesting form in Russia where there exists a language of sorts, most of its words based on four basic profane roots - nouns p...is, wh.0.e, c..t and verb f..k. It is possible to sensibly communicate using just these four basic roots.

21st September 2005, 01:31 PM	#6
VEC7OR diyAudio Member Join Date: Jul 2003 Location: Vilnius	I think I got it now, I should replace 100k resistor in input to 9k resistor, that way both inputs will see the ground through 10k resistance. __________________ Profanity took a very interesting form in Russia where there exists a language of sorts, most of its words based on four basic profane roots - nouns p...is, wh.0.e, c..t and verb f..k. It is possible to sensibly communicate using just these four basic roots.

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